It is well known in the art that a degree of opening of an air flow rate control door such as an air mixing door for use in adjusting a mixing ratio between a cold air cooled by an evaporator and a warm air heated by a heater core in an automobile air conditioning device is controlled in general by DC motor, and said motor is controlled by a control circuit including a calculation circuit for use in comparing and calculating a position signal transmitted from a position sensor circuit for said door and a target signal transmitted from a temperature setting device. As this control circuit, it has been proposed to provide a control circuit composed of a switching transistor shown, for example, in FIG. 1. In FIG. 1, reference 1 indicates a calculation amplifier in which a target signal V.sub.H is fed to a reverse input and a position signal V.sub.IN transmitted from a sensor circuit 3 for use in sensing a position of the air flow rate control door 2 is fed to a non-reverse input terminal via resistor 1a. The calculation amplifier 1 operates as a comparator, and its output terminal is connected to a base of PNP transistor 5 via resistor 4 and connected to a base of NPN transistor 9 via resistor 7 and diode 8. To the reverse input of the calculation amplifier 10 applied as a comparator is fed said position signal V.sub.IN via resistor 10a, and to the non-reverse input is fed a target signal V.sub.L. Output terminal of the calculation amplifier is connected to a base of PNP transistor 12 via resistor 11 and further connected to a base of NPN transistor 16 via resistor 14 and diode 15. Between the base and emitter of said transistor 5 is connected a resistor 17, and an emitter of said transistor is connected to a power supply, its collector is connected to a collector of the transistor 9, and further a collector of the transistor 5 is connected to a motor 18. Between a base and an emitter of said transistor 12 is connected a resistor 19, an emitter of said transistor is connected to the power supply, its collector is connected to a collector of the transistor 16, the collector is connected to another terminal of the motor 18. Between the base of the transistor 9 and an earth is connected a resistor 20, an emitter of the transistor 9 is connected to the earth. Between a base of the transistor 16 and the earth is also connected a resistor 21, and its emitter is connected to the earth. In this case, said target signal V.sub.H is set higher than the target signal V.sub.L. The motor 18 is applied to control a degree of opening of the door 2, for example, such as an air mixing door, and the position signal V.sub.IN for indicating a degree of opening of the door 2 is transmitted to each of the calculation amplifiers 1 and 10 by the opening degree sensor circuit 3 as the door 2 is controlled.
With the arrangement above, its operation will be described as follows. In case of V.sub.IN &gt;V.sub.H, an output from the calculation amplifier 1 becomes H-level, an output from the calculation amplifier 10 becomes L-level, thereby the transistors 12 and 9 are set ON and the remaining transistors are set OFF, so that an electric current a is flowed to the earth from the power supply via transistor 12, motor 18 and transistor 9, resulting in driving the motor. In turn, in case of V.sub.L &lt;V.sub.IN&lt;V.sub.H, since an output from the calculation amplifier 1 becomes L level, an output from the calculation amplifier 10 becomes L level, both transistors 5 and 12 are set ON and both transistors 9 and 16 are set OFF, resulting in that said electric current a flowed in the motor 18 is shut off and the motor 18 is then stopped. In case of V.sub.IN &lt;V.sub.L, the output from the calculation amplifier 1 becomes L level and the output from the calculation amplifier 10 becomes H level, so that both transistors 5, 16 are set ON, and the remaining transistors are set OFF, an electric current flowing in a direction opposite to that described above is flowed in the motor 18 and then the motor 18 is rotated in a reverse direction. Thus, the motor 18 is automatically controlled in such a manner as the position signal V.sub.IN is always set between the target signals V.sub.H and V.sub.L, and a position of the door 2 may be adjusted by providing a relative variation between the values of target signals V.sub.H and V.sub.L.
However, a conventional type of the control circuit for automobile electro-magnetic driving device shows that, as shown in FIG. 2, if an output from the calculation amplifier 1 is set at H level as shown in FIG. 2(a) before the time T.sub.1, the transistor 9 is also set ON as shown in FIG. 2(c) along with the transistor 12. From the time T.sub.1, an output from the calculation amplifier 1 begins to reverse to L level with a specified delay in response, then the transistor 9 is changed from its ON condition to its OFF condition as shown in FIG. 2(c), and the transistor 5 is also changed from its OFF condition to ON condition as shown in FIG. 2(b) substantially in synchroneous with the condition of the transistor 5.
Therefore, when the transistor 9 is set OFF, the transistor 5 is set ON, and at this time since a response speed i.e. a reversing speed of both transistors 5, 9 is low, resulting in producing a space S in which both transistors are set ON and during this time the electric current is flowed from the power supply to the earth via transistors 5, 9 to produce their short circuited condition. And further when a chattering operation is occurred at a time of reversing of the calculation amplifier 1, an apparent response of the amplifier is delayed and then said short circuited condition may be occurred. Therefore, a high electric current is flowed at the transistors 5 and 9, resulting in reducing a life of the elements. This condition may also be found in the transistors 12 and 16. Thus, it may also be assumed to connect a resistor 23 to a line 22 arranged between the emitters of the transistors 9, 16 and the earth so as to restrict the high electric current caused by said short circuited condition. However, in this case, a variation in voltage of power supply caused by a voltage drop across the resistor 23 is produced and a rotational speed of the motor is reduced. And with the control circuit composed of the elements described above, a similar problem in operation will be occurred when a battery damp serge 24 is flowed from the power supply for example when transistors 5, 6 are set OFF, transistors 9, 12 are set ON, respectively. That is, a voltage applied to the calculation amplifier 1 is usually restricted to such a value as being lower than a voltage of power supply under an influence of a protective resistor and capacitor connected to the calculation amplifier. Due to this fact, a value of voltage with H level transmitted from the output terminal of the calculation amplifier 1 becomes lower than the level of voltage at power supply. Since an emitter potential of the transistor 5 becomes lower than the base potential due to a presence of said battery damp serge 24, a conventional type of the control circuit shows such disadvantages as the transistor 5 is set ON and a short circuited current is flowed from the transistor 5 to the transistor 9 and both transistors are damaged thereby.